US President Barack Obama arrived in Cambodia last night having just won four more years in office, but that is nothing compared to his host, Hun Sen.

The Cambodian prime minister has held power since Ronald Reagan was in the White House, and says he is not stepping down until he is 90.

Hun Sen, 60, has a knack for making sure his rivals end up in jail or in exile. A laudatory biography is sub-titled Strongman of Cambodia, and some would say that was putting it mildly.

Yet, through his country's civil wars, a UN peace process and several elections, the one-time communist cadre has always managed to come out on top. Over the past decade, he has also overseen modest economic growth and stability in a country plagued by desperate poverty and nearly destroyed under the Khmer Rouge regime.

Obama's visit, the first by a United States president, comes because Cambodia is hosting the annual East Asia Summit.

These days, Hun Sen has styled himself as an elder statesman, and he is anxious to win international respectability to go along with the economic growth.

Cambodia has moved increasingly closer to China in recent years, but the US, too, has expanded its efforts to court its prime minister.

"He is intelligent, combative, tactical, and self-absorbed," says historian David Chandler, a expert on Cambodia at Australia's Monash University and a critic of Hun Sen's rule.

Despite concerns over his autocratic style and human rights lapses, the canny Hun Sen has managed to maintain the flow of the international aid that still accounts for much of Cambodia's national budget.

He also has apparent populist appeal. A poll of Cambodians taken in December by the International Republican Institute - the international democracy-promoting arm of the US Republican Party - found that 81 per cent of 2,000 respondents said Cambodia was "generally headed in the right direction".

But critics say that too often Hun Sen uses brute force and manipulation of the courts to maintain his hold on power.

Hun Sen's reputation for violence owes much to the fact that he staged a 1997 coup against his own coalition government. Forces loyal to him defeated those of his co-prime minister - whose party had actually won elections four years previously - putting Hun Sen once again in full control.

Opposition leader Sam Rainsy - the sole Cambodian politician with the charisma and resources to present any real challenge to Hun Sen's hold on the nation - is in self-imposed exile to avoid 12 years in prison from convictions critics say is political persecution.

Few doubt the outcome of the election next year.

As Hun Sen put it ahead of the last election in 2008: "I wish to state it very clearly this way: no one can defeat Hun Sen. Only Hun Sen alone can defeat Hun Sen."

This article appeared in the South China Morning Post print edition as Strongman aims to rule 'until he is 90'